dissymmetry of
liftDissymmetry of lift is the
difference in lift that exists between the advancing half of the rotor disk and
the retreating half. It is caused by the fact that in directional flight the
aircraft relative wind is added to the rotational relative wind on the advancing
blade, and subtracted on the retreating blade. The blade passing the tail and
advancing around the right side of the helicopter has an increasing airspeed
which reaches maximum at the 34 o'clock position. As the blade continues, the
airspeed reduces to essentially rotational airspeed over the nose of the
helicopter. Leaving the nose, the blade airspeed progressively decreases and
reaches minimum airspeed at the 9 o'clock position. The blade airspeed then
increases progressively and again reaches rotational airspeed as it passes over
the tail.
Note the shaded circle in the
picture labelled "REVERSE FLOW":
Blade airspeed at the outboard
edge of the shaded circle is 0 knots. Within the reverse flow area, the air
actually moves over the blade backwards from trailing edge to leading edge. From
the reverse flow area out to the blade tip, the blade airspeed progressively
increases up to 294 knots.
At an aircraft airspeed of 100
knots, a 200 knot blade airspeed differential exists between the advancing and
retreating blades. Since lift increases as the square of the airspeed, a
potential lift variation exists between the advancing and retreating sides of
the rotor disk. This lift differential must be compensated for, or the
helicopter would not be controllable.
To compare the lift of the
advancing half of the disk area to the lift of the retreating half, the lift
equation can be used. In forward flight, two factors in the lift formula,
density ratio and blade area, are the same for both the advancing and retreating
blades. The airfoil shape is fixed for a given blade. The only remaining
variables are changes in blade angle of attack and blade airspeed. These two
variables must compensate for each other during forward flight to overcome
dissymmetry of lift.
Two factors, rotor RPM
and aircraft airspeed, control blade airspeed during flight. Both factors
are variable to some degree, but must remain within certain operating limits.
Angle of attack remains as the one variable that may be used by the pilot to
compensate for dissymmetry of lift. The pitch angle of the rotor blades can be
varied throughout their range, from flat pitch to the stalling pitch angle, to
change angle of attack and to compensate for lift differential.
The following picture shows the
relationship between blade pitch angle and blade airspeed during forward flight:
Note that blade pitch angle is
lower on the advancing side of the disk to compensate for increased blade
airspeed on that side. Blade pitch angle is increased on the retreating blade
side to compensate for decreased blade airspeed on that side. These changes in
blade pitch are introduced either through the blade feathering mechanism or
blade flapping. When made with the blade feathering mechanism, the changes are
called cyclic feathering. Pitch changes are made to individual blades
independent of the others in the system and are controlled by the pilot's cyclic
pitch control.
Tail Rotor Dissymmetry of Lift
The tail rotor experiences
dissymmetry of lift during forward flight, because it also has advancing and
retreating blades. Dissymmetry is corrected for by a flapping hinge action. Two
basic types of flapping hinges, the delta and the offset hinge,
are used on most contemporary helicopters.
The delta hinge is not oriented
parallel to the blade chord:
It is designed so that flapping
automatically introduces cyclic feathering which corrects for dissymmetry of
lift. The offset hinge is located outboard from the hub:
The offset hinge uses
centrifugal force to produce substantial forces that act on the hub. One
important advantage of offset hinges is the presence of control regardless of
lift condition, since centrifugal force is independent of lift.
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